A Horny Proposition

Why do some female bovids have horns, whereas others don’t?

Males in the bovid family—antelopes, cattle, goats, sheep—have horns or antlers, which biologists agree evolved as weaponry to compete for mates. The origin of headgear in females, however, has remained enigmatic. Did they evolve in large bovids unable to easily hide or flee from predators? In group-living bovids with intense competition for food? Or in bovids whose females compete for territory?

The answer: none of the above—at least not completely. Two evolutionary biologists,Theodore Stankowich at the University of Massachusetts Amherst and Tim Caro at the University of California, Davis, studied the question in 117 species of bovids. Not until they considered how conspicuous a species is in its natural habitat did they find a strong correlation. Large species living in open habitats are visible to faraway predators; in almost every one, females have horns, presumably for defense. But in small species, or species living in brush or forest, females go bareheaded. In African duikers and a few other bovids, females fight each other for land; they, too, have horns.

The pattern holds for eighty of the eighty-two cases of horned females, and the researchers think it may apply to other hoofed mammals as well. (Proceedings of the Royal Society B)